What Causes Rust on Metals

Electrochemical corrosion is the most important
mechanism that causes Rust on Metals.

Four conditions must exist before electrochemical corrosion can proceed and metal rust:

(1) there must be something that rusts ,
the metal anode, where the
oxidation reaction takes place

(2) there must be a cathode, where the reduction reaction takes
place

(3) there must be continuous conductive liquid path (electrolyte, usually
condensate and salt or other contaminations), for example:
water, seawater, condensing water, humidity..

(4) there must be a conductor to carry the flow of electrons from the anode to
the cathode.

This conductor is usually in the form of metal-to-metal contact such as in
bolted or riveted joints.

The elimination of any one of the four conditions will remove
the conditions that causes rust on metal.
An unbroken (perfect) coating on the surface of the metal will prevent the
electrolyte from connecting the cathode and anode so the current cannot flow and rust on metal can't form.

Therefore, no rust will occur on metal
as long as the coating is
unbroken.